Integrated circuit designers use a variety of components to implement desired circuit functionality. These components may include bipolar and field-effect transistors, junction diodes, capacitors, and resistors.
Resistors are used for a wide variety of circuit applications in which resistance values are required to be quite large. For example, a resistor might be used to limit the current between its terminals for a given applied voltage. Such an applied voltage might be due to electrostatic discharge (ESD) which can damage integrated circuits. By choosing a large resistance value, the current produced by an ESD pulse of given voltage can be reduced, affording more protection to the integrated circuit. Resistors can also be used in reference circuits such as a bandgap voltage reference or as feedback elements in conjunction with operational amplifier circuits. Another application of the integrated circuit resistor is its use with an integrated circuit capacitor to form a characteristic time constant for signal frequency filtering applications. In order to pass very low frequencies, the desired product of the resistance and capacitance might be appreciable, demanding large resistance values. In low-power applications, large resistors are useful for limiting currents thereby reducing power consumption.
Integrated circuit resistors can be fabricated by the deposition of thin-film materials including nichrome or tantalum, but such implementations add process steps to a standard complementary metal-oxide-semiconductor (CMOS) process. Integrated circuit resistors can also be fabricated using standard CMOS processing steps. For example, resistors can be created from the polysilicon used to form the gate regions of metal-oxide-semiconductor field-effect transistors (MOSFETs), or from the diffused well regions in which MOSFETs are later created, or from the ion-implantation step used to create source and drain diffusion regions of MOSFETs.
Since implementing a resistor uses area on an integrated circuit die, it is desirable to increase the resistance obtained using a given circuit area without adding complexity to a standard CMOS process.